Can I hire a taxi from Nanning airport to Detian waterfall?

It may be hard to hire a taxi to that waterfall outside the airport.
The waterfall is about 210km from the airport and in another city. The local taxi drivers alway drive in their own cities and seldom to other places.
But you can try to negoiate with the taxi driver.
As i know, from local Langdong Bus Station, there is one direct bus to the waterfall at 08:30 and you may try that one.

Thanks for your advice. Do you know any website whereby I can book a car or taxi?

There is no such a website that you can prebook a taxi, so you may need to hire directly upon reaching the airport. It is easy to get one, so please do not worry about it.

Can I clarify if you mean I can negotiate with the can driver in airport to drive me to detian waterfall?

Yes, you can try to negotiate with the driver.